Electroluminescent devices

ABSTRACT

An electroluminescent device includes a piece of electroluminescent phosphor material of the kind having a localized region of high electrical resistivity produced by forming and means for applying across at least a part of the said region a series of unidirectional voltage pulses having a mean pulse height significantly greater than the forming voltage for the piece and a mean pulse length having such a value as to provide emission of light having a brightness corresponding to the upper part of the peak of the pulse response characteristic for the piece at that given mean pulse height. The piece is preferably a panel having a matrix of electroluminescent elements.

United States Patent 1191 Kirton et al.

[ Mar.4, 1975 ELECTROLUMINESCENT DEVICES [75] Inventors: John Kirton, Malvern; Adrian Leonard Mears, Cheltenham; Richard William Sarginson, Malvern; Norman John Werring, Dartford, all of England [22] Filed: May 22, 1973 [21] Appl. No.: 362,705

[30] Foreign Application Priority Data 313/108 A, 108 B, 483, 494, 498, 502; 252/301.6; 29/25.1l, 25.13;-l l7/33.5

[56] References Cited UNITED STATES PATENTS 2,972,692 2/1961 Thornton 313/108 A MEAN amcumess (FOOT LAMBERTS) o o lb PULSE LENGTH (/lsecs.)

2,972,694 2/1961 Thornton 313/108 A 3,185,893 5/1965 Sperling 313/108 A x 3,708,708 1/1973 Soxman 315/246 X Primary E.raminerAlfred E. Smith Assistant Examiner-E. R. LaRoche Attorney, Agent, or FirmElli0tt l. Pollock ABSTRACT An electroluminescent device includes a piece of electroluminescent phosphor material of the kind having a localized region of high electrical resistivity produced by forming and means for applying across'at least a part of the said region a series of unidirectional voltage pulses having a mean pulse height significantly greater than the forming voltage for the piece and a mean pulse length having such a value as to provide emission of light having a brightness corresponding to the upper part of the peak of the pulse response characteristic for the piece at that given mean pulse height. I

The piece is preferably a panel having a matrix of electroluminescent elements.

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cs OUTPUTTO H R4 v CONDUCTOR CONTROL LOGIC.

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snmsg 'g R6 R8 l PUT F om IL 2 OUTPUT To CONTROL I x CONDUCTOR LOGIC. TR3

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cheap manufacture. One such solid state display device is an electroluminescent device (panel) in which light is emitted from a region or a number of regions of electroluminescent phosphor material, such as zinc sulphide, by the direct application of an electric field across each region. The electric field applied across,

each regioncan either be alternating or unidirectional. The phosphor material is manufactured to suit the type of fields applied.

The present invention is concerned with an electroluminescent device having a phosphor material suitable for the application of unidirectional fields. Such material is characterized by the feature that it possesses a localized region of high electrical resistivity produced during its manufacture by a treatment known as forming. Any given piece of such material can be specified by the degree of forming applied to it. The degree of forming can be specified in relation to a forming voltage" (defined below) for the piece.

It has been discovered that if a series of unidirectional voltage pulses is applied across a formed piece of electroluminescent phosphor material the mean brightness of light emitted varies as a function of the mean length of the applied pulses. For'any given mean duty cycle (ratio of pulse length to time between corresponding points on adjacent pulses in the series) and any given mean pulse height (magnitude) this function can be uniquely defined for the piece.

The function will, for a given duty cycle and a given pulse height, be herein referred to as the pulse response characteristic of the piece. Each pulse response characteristic is most meaningful when plotted on a graph in which both axes are logarithmic.

The pulse response characteristic for mean pulse heights equal to, close to or less than the forming voltage for the piece are basically plateau-shaped (when plotted logarithmically). In other words, the mean brightness obtained does not differ significantly from that obtained by essentially continuous operation, except for pulses having a short pulse length where the mean brightness is significantly less.

We have found, however, that the pulse response characteristic for mean pulse heights significantly greater than the forming voltage for thepiece possesses (when plotted logarithmically) a smooth, well-defined peak for pulses whose mean pulse length falls within a given band or region (occurring at short mean pulse lengths). We have found, unexpectedly, that it is beneficial to use pulses having a mean pulse length such as to provide emission of light having a brightness corresponding to the upper part of this peak.

According to the present invention there is provided an electroluminescent device including a piece of elec troluminescent phosphor material of the kind having a localized region of high electrical resistivity produced by forming and means for applying across at least a part of the said region a series of unidirectional voltage pulses having a mean pulse height significantly greater than-the forming voltage for the piece as hereinafter defined and a mean pulse length having such a value as to provide emission of light having a brightness corresponding to the upper partof the peak of the pulse response characteristic as herein defined for the piece at that given mean pulse height.

According to an aspect of the present invention there is provided an electroluminescent device including a piece of electroluminescent phosphor material of the kind having a localized region of high electrical resistivity produced by forming and, applied across at least a part of the said region, a series of unidirectional voltage pulses having a mean pulse height significantly greater than the forming voltage for the piece as hereinafter defined and a mean pulse length having a value provid ing emission of light having a brightness corresponding to the upper part of the peak of the pulse response characteristic as herein defined for the piece at that given mean pulse height.

According to another aspect of the present invention there is provided an electroluminescent device including a plurality of elements of electroluminescent phosphor material arranged in matrix formation each of the kind having a localized region of high electrical resistivity produced by forming, means for applying a first series of unidirectional electrical potential pulses of one polarity to a selected row of elements in the matrix and means for applying a second series of unidirectional electrical potential pulses of the opposite polarity to a selected column of elements in the matrix, the pulses of the first series being contemporaneous with those of the second series, the pulses of the first and second series having respective mean pulse heights such that the sum of their magnitudes is significantly greater than the forming voltage as hereinafter defined for the elements in the matrix and the pulses of both series having a pulse length having such a value as to provide emission of light having a brightness corresponding to the upper part of the peak of the pulse response characteristic as herein defined for the elements in the matrix at that given mean pulse height.

The elements can either be separate pieces or parts of a single piece. 1

Where the term a piece" is used in this specification and the appended claims in relation to electroluminescent phosphor material, the term is to be construed as including a layer of powdered electroluminescent phosphor material held in a binding medium, and a thin film of evaporated electroluminescent phosphor mate rial.

The. upper part" of the peak of a pulse response characteristic of a given region or element of electroluminescent phosphor material will be defined for the purposes of this specification and the appended claims as the region on that characteristic where the brightness B of the emitted light is equal to or greater than a given brightness B The brightness B will be defined as follows. If the brightness of light obtained when operating at the maximum of the peak of the pulse response characteristic is B and the brightness of light obtained when operating on the pulse response characteristic with pulses having a pulse length of 5 milliseconds (which is essentially the same as continuous operation) is B then the brightness B is given by the equation log B 1%(log B log B In other words the upper part of the peak is that region above half-height when the pulse response characteristic is plotted logarithmically.

In absolute terms B is hence given by the equation Embodiments of the present invention will be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a graph consisting of pulse response characteristics for a given piece of electroluminescent phosphor material.

FIG. 2 is a part cross-sectional elevation of a simple electroluminescent panel which may be operated in a manner embodying the present invention.

FIG, 3 is a part cross-sectional plan on the line III- III of the panel of FIG. 2.

FIGS. 4, 5, 6 and 7 are circuit diagrams of different circuits which can be used to apply electrical potential pulses to the conductors of electroluminescent panels (such as that of FIG. 2 and FIG. 3).

United Kingdom Patent SpecificationNo 1,300,548 (Vecht U.S. Pat. No. 3,731,353) describesa method of manufacturing an electroluminescent phosphor material suitable for use with the application of unidirectional electric fields, i.e. suitable for use in connection with the present invention. Basically, the method is as follows. An admixture is formed of particles of a com-- pound or compounds of an element ofGrou'p IIb with an element of group Vlb (such as zinc sulphide) and an activator such as manganese. The particles of the admixture are then coated with an element of group lb- (such as copper). The coated particles are embedded in a translucent binding matrix (such as -polymethylmethacrylate) to form a piece (normally a layer) of phosphor material. An electrode is attached to one part of the piece and another electrode is attached to another part of the piece. A unidirectional voltage isapplied between the electrodes. This produces an electrical'current in'the piece of phosphor material. This current produces a localized region of high electrical resistivity within the piece of phosphor material near the positive electrode.

Alternatively, suitable material may be provided by the deposition on a suitable substrate such as glass of a thin film of phosphor material. by evaporation. A 10- calized region of high electrical resistivity is then formed in the film in a manner similar to that described above.

In any method of producing a piece of phosphor material suitable for operation using unidirectional voltages (including the methods described above), the most essential step is that of producing the localized region of high electrical resistivity. It is from thisregion that light emission occurs during operation of a device made from the piece of material. The step of producing the region is known as forming.

Forming may be carried out by the application of a steady voltage (typically, although not essentially, 25 volts) between the electrodes for a short period (typically, although not essentially, 2 or 3 minutes) until the piece of phosphor material is capable ofweakly emitting light (at a level of about 5 l0 foot-lamberts), followed by the application of a steadily increasing voltage between the electrodes at an approximately constant power (typically, although not essentially, 2 watts per sq cm) through the piece of phosphor material over 7 a longer period (typically, although not essentially an hour) until a maximum voltage is reached. This type of forming is known as dc forming at approximately constant power.

Forming may, however, be carried out in other ways such as by the application of voltage pulses or by the application of a steadily increasing voltage followed by a voltage step. However, a piece of phosphor material formed in another way will have pulse response characteristics equivalent to those of a particular corresponding piece formed by dc forming at approximately constant power.

For the purposes of this specification and the appended claims the forming voltage of a given piece or element of electroluminescent phosphor material will be defined as follows. It is the maximum voltage used to form the given piece or. element by dc forming at approximately constant power. Alternatively, in the case of a given piece or element formed otherwise than by dc forming at approximately constant power, it is the maximum voltage which would be-used to form by dc forming at approximately constant power a piece of phosphor material having equivalent pulse response I characteristics to those of the given piece or element.

FIG. 1 is a graph consisting of pulse response characteristics for a given piece of electroluminescent phos-, phor material having a forming voltage of about volts. The axes of the graph, which consist of the mean brightness of emitted light in foot-lamberts and the mean pulse length in microseconds, are both on logarithmic scale. The characteristics are plotted for respective pulse heights of 40, 50, 70, 90 and 110 volts and for a duty cycle of-O.5 percent.

When the mean voltage pulse height (magnitude) i.e., the average height of a series of applied'voltage pulses, is close to or below the forming voltage (i.e. is

either 40 volts or 50 volts) the characteristic is approxi-' matelyplateau-shaped. However, at the higher values of voltage pulse height (magnitude) (i.e. 70, 90 and l 10 volts), a definite peak isdevelopedin the characteristic. It has unexpectedly been found beneficial to operate devices made from a piece of formed electroluminescent phosphor material using a series of pulses 1 having a length (typically 2 p. sec) corresponding to the uppe'rpart of the pulse response characteristic for that series of pulses above the forming voltage.

In order to achieve emission of light having a highbrightness from an electroluminescent device of the type described without the use of latching elements it would normally be considered necessary to operate the device using a series of unidirectional voltage pulses. However, it would be expected that it would be very harmful to the device to use pulses having a mean height (magnitude) significantly greater than the forming voltage and having short pulse length. This is because the peak (pulse) power delivered to the device in such an arrangement is much greater than that for essentially continuous operation using the same mean power. However, in connection with the present invention, it has unexpectedly been found that the use of a series of pulses having a mean pulse height significantly greater than the forming voltage and a mean pulse length (i.e., the average length of the pulses in the series of applied pulses) corresponding to operation on the upper part of the peak of the pulse response characteristic for that series of pulses is not harmful to the device. In fact the active life of the device is very much longer than expected, and the level of brightness of emitted light remains essentially constant for the major part of the life (whereas it continuously falls using essentially continuous operation). Furthermore, besides providing improved brightness, the arrangement also provides improved discrimination between the brightness of light emitted from regions of a given piece of electroluminescent material operated (lit) and those not intended to be operated (lit).

FIG. 2 is a part cross-sectional elevation, and FIG. 3 is a part cross-sectional plan on the line IIIIII in FIG. 2, of a simple electroluminescent panel which may be operated in a manner embodying the present invention. A block 1 made of glass bears a strip 3 made of transparent electrically conducting material such as tin oxide and a strip 5 parallel with the strip 3 and made of the same material. A layer 7 of formed electrolumines'cent material produced by one of the methods described above is deposited on the block 1 over the strips 3, 5. A strip 9 of conducting material such as aluminum is deposited on the surface of the layer 7 so as to run in a direction perpendicular to the strips 3, 5. A strip 11 of the same conducting material is deposited on the surface of the layer 7 parallel to the strip 9. A strip 13 of conducting material such as aluminum and a strip 15 of the same material are deposited on the g block 1 parallel to the strips 9, 11. The strip 9 is electrically connected tothe strip 13 by aregion 17 of conducting paint applied at the edge of the layer 7, and the strip 11 is electrically connected to the strip 15 by a region of electrically conducting paint applied at the edge of the layer 7. An encapsulating cover 21, made for example of resin, is deposited on the block 1 preferably while in an inert atmosphere and around thelayer- 7. The cover 21 prevents atmospheric contaminants from reducing the active life of the layer 7. The strips 3, 5, l3, 15 each have an end protruding outside the cover 21. The protruding end of the strip 3 is connected to an external conductor Y1. Likewise, the protruding ends of the strips 5, 13 and 15 are respectively connected to external conductors Y2, X1 and X2.

When a unidirectional operating voltage is applied between one of the conductors X1, X2 (which is made negative) and one of the conductors Y1, Y2 the voltage causes emission of light from the region of the layer 7 between the corresponding strips to which those conductors are electrically connected and the light is observed to pass through the block 1. For example, if a unidirectional operating voltage is applied between the conductor X1 and the conductor Y1 (with the conductor Y1 made positive) the voltage appears across the layer 7 in the region where the strip 3 and the strip 9 overlap.

In accordance with an embodiment of the invention,

the operating voltage applied are pulses having amean pulse height (magnitude) greaterthan the magnitude of the forming voltage for the layer 7 and having a mean pulse length corresponding to the upper part of the peak of the pulse response characteristic for the layer 7 and for those given pulses.

The others will respectively include the regions of the layer 7 where the strip 3 and the strip 11 overlap, where the strip 5 and the strip 9 overlap and where the strip 5 and the strip 11 overlap.

The regions of the layer 7 where the strip 3 and the strip 9 overlap, where the strip 3 and the strip 11 overlap, where the strip 5 and the strip 9 overlap and the strip 5 and the strip 11 overlap will hereinafter be referred to collectively as EL (electroluminescent) regions. The conductors Y1, Y2 will hereinafter be referred to collectively as Y conductors. The conductors X1, X2 will hereinafter be referred to collectively as X conductors.

There are various addressing methods by which voltage pulses can be applied to thepanel described with reference to FIG. 2 and FIG. 3, or to any similar panel containing a greater number of EL regions appropriately connected to corresponding X conductors and Y conductors. Suppose that it is necessary to apply a voltage V volts across certain EL regions of a given panel to produce light emission from those regions.

A first addressing method is as follows. The X conductors are held at ground potential. A positive pulse having a magnitude V volts is applied in turn to each Y conductor. The disadvantage of this method is that the voltage V is applied across every EL region electrically connected to the same Y conductor.

A second addressing method is as follows. The X conductors and the Y conductors are all initially held at ground potential. A first positive potential pulse having a magnitude V1 is applied to a first Y conductor. A first negative potential pulse, having a magnitude V2, is applied contemporaneously with the first positive pulse, to each X conductor electrically connected to an EL region which it is desired to operate (and which region is also electrically connected to the first Y conductor). The magnitudes of V1 andVZ are such that [V2] {V2}= [V]. The values of these magnitudes mayle optimized by theory or experimentation. The first positive pulse and the first negative pulse are then contemporaneously removed. After a gap in time, a second positive potential pulse (identical with the first positive pulse) is then applied to the next Y conductor. A second negative potential pulse (identical with the first negative pulse) is applied, contemporaneously with the second positive pulse, to each X conductor electrically connected to an EL region which it is desired to operate (and which region is also electrically connected to the Y conductor to which the second positive pulse is applied). The second positive pulse and the second negative pulse are then contemporaneously removed. If the panel contains a matrix having more than 2 X 2 EL regions, a third positive potential pulse, identical with the first and second positive pulses, is applied to the next Y conductor after a gap in time, while a third negative potential pulse, identical with the first and second negative pulses, is applied, contemporaneously with the third positive pulse, to appropriate X conductors, and so on. When a pulse has been applied across every appropriate EL region in the panel the method is repeated to provide refreshed operation.

FIG. 4 is a circuit diagram of a circuit which can be used to apply a positive potential pulse to a Y conductor by the second addressing method described above. An input conductor C1 leading from control logic (not shown) is connected to one end of a resistor R1. The other end of the resistor R1 is connected to the base of an n-p-n transistor TRl. The resistor R1 and the base of the transistor TRl have a common connection to one end of a resistor R2, the other end of which is connected to a grounded conductor C3. The emitter of the transistor TR1 is also connected to the conductor C3. The collector of the transistor TRl is connected to one end of a resistor R3, the other end of which is connected to an HT conductor C2 (having a potential of (about) V1 volts). The collector of the transistor TRl and the resistor R3 also have a common connection, consisting of a conductor C4, to the base of an n-p-n transistor TR2, whose collector is connected to the conductor C2 and whose emitter is connected to one end of a resistor R4. The other end of the resistor R4 is connected to the conductor C3. A unidirectionally conducting diode D1 is connected between the emitter of the transistor TR2 and the conductor C4, in such a way as to conduct whenever the emitter of the transistor TR2 is positive with respect to the conductor C4. An output conductor C5 is also connected to the emitter of the transistor TR2. The conductor C5 leads to a Y conductor (not shown in FIG. 4) of an electroluminescent panel (such as the conductor Y1 in the panel described with reference to FIG. 2 and FIG. 3).

The circuit operates as follows. The transistor TR1 is initially in its ON (conducting) state and the transistor TR2 is initially in its OFF (non-conducting) state. The conductor C1 is initially at a small positive potential (about 5 volts). The conductor C5 is initially at ground potential. An input control pulse is supplied from the control logic. This reduces the potential of the conductor C1 to ground potential and causes the transistor TRl to be switched into its OFF state. The potential of the collector of the transistor TRl therefore rises, causing a corresponding rise in the potential of the base of the transistor TR2. This causes the transistor TR2 to be switched into its ON state. A currentpath is therefore formed from the conductor C2 to the conductor C5 through the transistor TR2, and this allows the potential of the conductor C5 to rise to about 1 voltbelow that of the potential of the conductor C2. At the end of the control pulse the potential. of the conductor C1 returns to its initial value. The transistor TRl is switched back into its ON state. The transistor TR2 is switched back into its OFF state, and the potential of the conductor C5 is therefore reduced. As'soon as the transistor TR2 is in its OFF state the diode D1 is forward biased. A current path therefore exists between the emitter of the transistor TR2 and the conductor C3, through the diode D1 and the transistor TRl. The positive side of theEL region (not shown in FIG. 4) to 'used to apply a negative potential pulse to an X conductor by the second addressing'method described above. The circuit is the p-n-p transistor analogue of that described with reference to FIG. 4. An input conductor C6 leading from control logic (not shown) is connected to one end of a resistor R5. The other end of the resistor R5 is connected to the base of a p-n-p emitter of the transistor TR3 is also connected to the conductor C8. The collector of the transistor TR3 is connected to one end of a resistor R7, the other end of which is connected to a conductor C7 held at apotential of about V2 volts. The collector of the transistor TR3 and theresistor R7 also'have a common connection, consisting of a conductor C9, to the base of a p-n-p transistor TR4, whose collector is connected to the conductor C7 and whose emitter is connected to one end of a resistor R8. The other end of the resistor R8 is connected to the conductor C8. A unidirectionally conducting diode D2 is connected between the emitter of the transistor TR3 and the conductor C9, in such a way as to conduct whenever the conductor C9 is positive with respect to the emitter of the transistor TR4. An output conductor C10 is also connected to the emitter of the transistor TR3. The conductor C10 leads to an X conductor (not shown in FIG. 4) of the electroluminescent panel (such as the conductor X1 in the panel described with reference to FIG. 2 and FIG. 3).

The circuit operates as follows. The transistor TR3 is initially in its ON state, and the transistor TR4 is ini-' tially in its OFF state. The conductor C6 is initially at ground potential, and the conductor C10 is initially at i the potential of the conductor C8. An'input control pulse is supplied from the control logic. This raises the potential of the conductor C6 to about +5 volts. The transistor TR3 is then switched into its OFF state. The potential of the collector of the transistor TR3 therefore falls, causing a corresponding fall in the potential of the base of the transistor TR4. This causes the transistor TR4 to be switched into its ON state. A current path is therefore formed from the conductor C7 to the conductor C10 through the transistor TR4, and this allows the potential of the conductor C10 to fall to about '1 volt above that of the conductor C7. At the end of the control pulse'the potential of the conductor C6 returns to that of ground. The transistor TR3 is therefore switched back into its ON state. The transistor TR4 is consequently swit'chedback into its OFF state, and the potential of the conductor C10 therefore rises. As soon as the transistor TR4 isin its OFF state again the diode D1 is forward biased. A current path therefore exists between the emitter of the traNsistor TR4 and the conductor C8, through the diode D2 and the transistor TR3. The negative side of the EL region (not shown in FIG. 5) to which the conductor C5 is connected is therefore quickly discharged via this current path.

For an EL region having a light-emitting area of about 4mm the transistors TR3 and TR4 may suitably be 2N540l transistors, the resistors R5, R6, R7, R8 may suitably have respective values of IKQ, 3309, 3.3KQ and 33KQ and the diode D2 may be an AAY12 diode. I

FIG. 6 is a circuit diagram of a simpler circuit (than that described with reference to FIG. 4) which can be used to apply a positive pulse'to a Y conductor by the second addressing method described above. An input conductor C11 leading from control logic (not shown) is connected to one plate of a'capacitor CAPl. The other plate of the capacitor CAPl is connected to the base of a p-n-p transistor TR5. The base of the transistor TRS is also connected to one end of a resistor R9,

. 9 the other end of which is connected to an HT conductor C12 held at a potential of about +V1 volts. The emitter of the transistor is also connected to the conductor C12. The collector of the transistor TRS is connected to one end of a resistor R10, the other end of which is connected to a grounded conductor C13. The collector of the transistor TR5 is also connected to a conductor C14 leading to a Y conductor (not shown in FIG. 6) of an electroluminescent panel.

The circuit operates as follows. The transistor TRS is initially in its OFF state. The conductor'Cll is initially at a small positive potential (about 5 volts) and the conductor C14 is initially at ground potential. A control pulse is supplied by the control logic. This causes the potential of the conductor C11 to fall to ground potential. The potential of the base of the transistor TRS consequently falls to a potential about 5 volts below that of the conductor C12, and the transistor TR5 is thereby switched into its ON state. A current path then exists between the conductor C12 and the conductor C14 through the transistor TRS. This path allows the potential of the conductor C14 to rise to about I volt below that ofthe conductor C12. At the end of the control pulse the potential of the conductor C11 rises to its initial value. The transistor TR5 is switched into its OFF state, and the potential of the conductor C14 falls again. The positive side of the EL region (not shown) to which the conductor C14 is connected is discharged through the resistor R10. The time constant of the dis charge is proportional to the product of the resistance of the resistor R10 with the capacitance of the EL region. The capacitor CAPl is'included in the circuit to isolate the base of the transistor TRS which is always within about 5 volts of the potential of the conductor C12 from the control logic; otherwise the control logic would be damaged by such a large potential difference.

For an EL region having a light-emitting area of 4mm, the transistor TRS may suitably be a 2N5401 transistor, the capacitor CAPl may suitably be a 0. l ,u.F capacitor and the resistor R9 and the resistor R10 may suitably be a 6800 resistor and an 8.21). resistor respectively.

FIG. 7 is a circuit diagram of a simpler circuit (than that described with reference to FIG. 5) which can be used to apply a negative pulse to an X conductor by the second addressing method described above. An input conductor C17 leading from control logic (not shown) is connected to the base of an n-p-n transistor TR6. Thebase of the transistor TR6 is also connected to one end of a resistor R11, the other end of which is connected to an HT conductor C15 held at a negative potential of about V2 volts. The emitter of the transistor TR6 is also connected to the conductor'ClS. The collector of the transistor TR6 is connected to one end of a resistor R12, the other end of which is connected to a conductor C14 held at a small positive potential (about 5 volts). The collector of the transistor TR6 is also connected to a conductor C16 leading to an X conductor (not shown in FIG. 7) of the electroluminescent panel. The circuit operates as follows. Initially, the transistor TR6 is in its OFF state, the conductor C17 is at ground potential and the conductor C16 is at the potential of the conductor C14. A control pulse is supplied by the control logic. This-causes the potential of the conductor C17 to rise to about +5 volts. The potential of the base of the transistor TR6 consequently rises, and the transistor TR6 is switched into its ON state. A current path then exists between the conductor C15 to the conductor C16 through the transistor TR6. This allows the potential of the conductor C16 to fall to about 1 volt above that ofthe conductor C15. At the end of the control pulse the potential of the conductor C17 returns to ground potential. The transistor TR6 is switched into its OFF state, and the potential of the conductor C16 rises. The negative side of the EL region (not shown) I to which the conductor C16 is connected is discharged through the resistor R12. The time constant of the discharge is proportional to the product of the resistance of the resistor R12 with the capacitance of the EL region. The capacitor CAP2 is included to isolate the base of the transistor TR6 from the control logic.

For an EL region having a light emitting area of 4mm", the transistor TR6 may suitably be a 2N555l transistor, the capacitor CAP2 may suitably be a 0. l ,u.F capacitor, and the resistor R11 and the resistor R12 may suitably be a 6809 resistor and an 8.2KQ resistor respectively.

A separate circuit for applying a positive potential pulse (such as that described with reference to FIG. 4 or that described with reference to FIG. 6) is normally connected to each individual Y conductor of a given panel, while a separate circuit for applying a negative potential pulse (such as that described with reference I to FIG. 5 or that described with reference to FIG. 7) is normally connected to each individual X conductor of that panel.

In the embodiments of the invention described above the duty cycle used in operation is not necessarily determined by the size of the panel. It has been discovered that the brightness obtained from each element is a plateau-shaped function (when plotted logarithmically) of the duty cycle used for. a given forming voltage. Therefore for the elements of a panel formed at a given forming voltage the duty cycle is chosen so that the brightness obtained is near the top of the plateau. Typically, although not essentially, the duty cycle is 1 percent or less.

We claim:

1. An electroluminescent device suitable foruse with the application of unidirectional electric fields comprising a piece of electroluminescent phosphor material of the kind having a localized region of high resistivity produced by forming, andrneans for applying across at least a part of said region a series of unidirectional voltage pulses, said pulses having pulse heights whose average for the series is significantly greater than the forming voltage for the piece and pulse lengths whose average for the series is a value operative to provide emission of light having a brightness corresponding to the upper part of the peak of the pulse response characteristic for the piece at said average pulse height.

2. An electroluminescent device as claimed in claim 1 including at least first, second, third and fourth regions of electroluminescentphosphor material of said kind having a localized region of high electrical resistivity produced by forming, at least first, second, third and fourth conductors, said first conductor being attached to a first part of said first region and to a first part of said second region, said second conductor being at tached to a first part of said third region and to a first part'of said fourth region, said third conductor being attached to a second part of said first region and to a second part of said third region, and said fourth conductor being attached to a second part of said second and the third conductor, the first conductor and the fourth conductor, the second conductor and the third conductor and the second conductor and the fourth conductor, said applied voltages being in the form of a vseries of unidirectional voltage pulses having said average pulse height for said first, second, third and fourth regions of electro-luminescent phosphor material and having said average pulse length for said first, second, third and fourth regions of electroluminescent material at said average pulse height.

3. An electroluminescent device as claimed in claim 2 wherein the means for applying voltages is operative to cause the voltage pulses applied between said first conductor and at least one conductor belonging to the pair consisting of said third conductor and said fourth conductor to be applied during the time intervals when the voltage pulses applied between said second conductor and at least one conductor belonging to the pair consisting of said third conductor and said fourth conductor are removed, and is operative to cause the voltage pulses applied between said second conductor and at least one conductor belonging to the pair consisting of said third conductor and, said fourth conductor to be applied during the time intervals when the voltage pulses applied between said first conductor and said at I least one conductor belonging to the pair consisting of said third conductor and said fourth conductor are removed.

4. An electroluminescent device as claimed in claim 3 wherein the voltage pulses applied by the means for applying voltages consist of electrical potential pulses of one polarity applied to said first conductor and electrical potential pulses of the opposite polarity contemporaneously applied to said at least one conductor belonging to the pair consisting of said third conductor and said fourth conductor, and consist of electrical potential pulses of said one polarity applied to said second conductor and electrical potential pulses of said opposite polarity contemporaneously applied to said at least one conductor belonging to the pair consisting of said third conductor and said fourth conductor.

5. An electroluminescent device according to claim 4 wherein said means for applying voltages is operative to cause each of the electrical potential pulses applied to any one of said first, second, third and fourth conductors to be applied only after a period when the magnitude of the electrical potential of that one conductor has fallen to a low value compared with the magnitude of the last electrical potential pulse in the series applied to that one conductor.

6. An electroluminescent device as claimed in claim 2, and wherein said regions of electroluminescent phosphor material are arranged in rows and columns in a matrix formation and are part of an electroluminescent panel.

7. An electroluminescent device as claimed in claim 4, and wherein said regions of electroluminescent phosphor material are arranged in rows and columns in a matrix formation and are part of an electroluminescent panel.

8. An electroluminescent device according to claim 4 wherein the means for applying voltages includes at least one switching circuit for switching one of said first, second, third and fourth conductors between a condition in which the magnitude of the electrical potential of that one conductor is high and a condition in which the magnitude of the electrical potential of that one conductor is low, the switching circuit incorporating at least one switching transistor responsive to low voltage control signals.

9. An electroluminescent device as claimed in claim 1 and wherein said series of unidirectional voltage pulses has a duty cycle as hereinbefore defined of at most 1 percent. 

1. An electroluminescent device suitable for use with the application of unidirectional electric fields comprising a piece of electroluminescent phosphor material of the kind having a localized region of high resistivity produced by forming, and means for applying across at least a part of said region a series of unidirectional voltage pulses, said pulses having pulse heights whose average for the series is significantly greater than the forming voltage for the piece and pulse lengths whose average for the series is a value operative to provide emission of light having a brightness corresponding to the upper part of the peak of the pulse response characteristic for the piece at said average pulse height.
 2. An electroluminescent device as claimed in claim 1 including at least first, second, third and fourth regions of electroluminescent phosphor material of said kind having a localized region of high electrical resistivity produced by forming, at least first, second, third and fourth conductors, said first conductor being attached to a first part of said first region and to a first part of said second region, said second conductor being attached to a first part of said third region and to a first part of said fourth region, said third conductor being attached to a second part of said first region and to a second part of said third region, and said fourth conductor being attached to a second part of said second region and to a second part of said fourth region, and means for applying voltages between at least one pair belonging to the pairs consisting of the first conductor and the third conductor, the first conductor and the fourth conductor, the second conductor and the third conductor and the second conductor and the fourth conductor, said applied voltages being in the form of a series of unidirectional voltage pulses having said average pulse height for said first, second, third and fourth regions of electro-luminescent phosphor material and having said average pulse length for said first, second, third and fourth regions of electroluminescent material at said average pulse height.
 3. An electroluminescent device as claimed in claim 2 wherein the means for applying voltages is operative to cause the voltage pulses applied between said first conductor and at least one conductor belonging to the pair consisting of said third conductor and said fourth conductor to be applied during the time intervals when the voltage pulses applied between said second conductor and at least one conductor belonging to the pair consisting of said third conductor and said fourth conductor are removed, and is operative to cause the voltage pulses applied between said second conductor and at least one conductor belonging to the pair consisting of said third conductor and, said fourth conductor to be applied during the time intervals when the voltage pulses applied between said first conductor and said at least one conductor belonging to the pair consisting of said third conductor and said fourth conductor are removed.
 4. An electroluminescent device as claimed in claim 3 wherein the voltage pulses applied by the means for applying voltages consist of electrical potential pulses of one polarity applied to said first conductor and electrical potential pulses of the opposite polarity contemporaneously applied to said at least one conductor belonging to the pair consisting of said third conductor and said fourth conductor, and consist of electrical potential pulses of said one polarity applied to said second conductor and electrical potential pulses of said opposite polarity contemporaneously applied to said at least one conductor belonging to the pair consisting of said third conductor and said fourth conductor.
 5. An electroluminescent device according to claim 4 wherein said means for applying voltages is operative to cause each of the electrical potential pulses aPplied to any one of said first, second, third and fourth conductors to be applied only after a period when the magnitude of the electrical potential of that one conductor has fallen to a low value compared with the magnitude of the last electrical potential pulse in the series applied to that one conductor.
 6. An electroluminescent device as claimed in claim 2, and wherein said regions of electroluminescent phosphor material are arranged in rows and columns in a matrix formation and are part of an electroluminescent panel.
 7. An electroluminescent device as claimed in claim 4, and wherein said regions of electroluminescent phosphor material are arranged in rows and columns in a matrix formation and are part of an electroluminescent panel.
 8. An electroluminescent device according to claim 4 wherein the means for applying voltages includes at least one switching circuit for switching one of said first, second, third and fourth conductors between a condition in which the magnitude of the electrical potential of that one conductor is high and a condition in which the magnitude of the electrical potential of that one conductor is low, the switching circuit incorporating at least one switching transistor responsive to low voltage control signals.
 9. An electroluminescent device as claimed in claim 1 and wherein said series of unidirectional voltage pulses has a duty cycle as hereinbefore defined of at most 1 percent. 